New approaches to the study of bulbospinal (B3) serotonergic neurons in the control of blood pressure

The technique of in vivo brain dialysis, recently described by Ungerstedt et al. (1982) provides an opportunity for the direct collection, identification and measurement of neurotransmitters released by activation of a particular pathway. Whereas Ungerstedt et al. used the method for the study of dopamine release by nigrostriatal neurons, it is applied here for the first time to the study of serotonin released by descending spinal neurons regulating sympathetic activity and in turn blood pressure. The first set of experiments performed were designed to test the hypothesis, arising out of previous experiments in the authors' laboratory, that bulbospinal serotonin neurons can exert a pressor effect through release of serotonin in the intermedio-lateral cell column. Micro-injections of kainic acid were made into the area of the lateral B3 serotonin cell group in the medulla. This elicited an increase in the release of serotonin in the spinal cord, measured using in vivo dialysis, accompanied by an increase in blood pressure. Pretreatment with 5.7-dihydroxytryptamine (5.7DHT) 2 weeks earlier completely prevented the increase in serotonin release and in blood pressure evoked by micro-injection of kainic acid into the B3 serotonin cells. These experiments used the technique of brain dialysis to support the hypothesis that bulbospinal serotonin nerves can exert a pressor action. In a second set of experiments L-glutamate was injected into the region of the lateral B3 serotonin cells near to the ventral surface of the medulla, and also into the midline B3 serotonin cells in the raphe, in order to activate neuronal cell bodies without stimulating fibres of passage.